When an Outlook 2010 account is configured to use Cached Exchange Mode, Outlook 2010 works from a local copy of a user's Microsoft Exchange mailbox stored in an offline data file (.ost file) on the user's computer, together with the Offline Address Book (OAB). The cached mailbox and OAB are updated periodically from the Exchange Server computer.

Cached Exchange Mode was introduced in Outlook 2003 to provide users a better online and offline experience. Cached Exchange Mode lets users move between connected and disconnected environments without interrupting their experience in Outlook. Also, it insulates users from network latency and connectivity issues while they are using Outlook.

In contrast, Online Mode works directly by using information from the server. When new information is required in Outlook, a request is made to the server and the information is displayed. Mailbox data is only cached in memory and never written to disk.

Cached Exchange Mode or Online Mode can be selected by the user during account setup or by changing the account settings. The mode can also be deployed by using the Office Customization Tool (OCT) or Group Policy.

Important

There is a known issue in which an additional Exchange account is added to the Outlook profile when a user who already has an exchange account in the profile is upgraded from Outlook 2003 or Outlook 2007. This issue can occur while you are upgrading Outlook and applying customizations by using a custom OCT file (.msp) or .prf file that is configured to “Modify Profile" and "Define changes to make to the existing default profile.”
To prevent multiple Exchange accounts from being created in one profile when you upgrade users to Outlook 2010, you must download and use the Service Pack (SP1) version of the OCT, which is available from the Microsoft Download Center (http://go.microsoft.com/fwlink/p/?LinkId=189316). To update the OCT, replace the /Admin folder that is in your Office 2010 installation files or installation image with the new /Admin folder that is included in the download package. If you do not use the SP1 version of the OCT, you must create a .prf file and set the properties BackupProfile=False and UniqueService=Yes. For the steps to do this, see Multiple Exchange accounts created in Outlook 2010 with existing Outlook profiles after upgrading from an earlier Office version using a custom MSP (http://go.microsoft.com/fwlink/p/?LinkId=199704).

Online Mode is the legacy method of connecting to Microsoft Exchange. It is a fully supported configuration in Office Outlook 2003, Outlook 2007, and Outlook 2010. Online Mode has value in certain scenarios in which the behavior of Cached Exchange Mode is unwanted. Example scenarios include the following:

“Kiosk” scenarios in which a particular computer has many users who access different Outlook accounts and the delay to download e-mail messages to a local cache is unacceptable.

Heavily regulated compliance or secure environments in which data must not be stored locally for any reason. In these environments, we recommend that you evaluate Encrypting File System (EFS) or BitLocker in addition to Cached Exchange Mode as a potential solution.

Very large mailboxes on computers that do not have enough hard disk space for a local copy of the mailbox.

Very large mailboxes (greater than 25 GB) on which performance considerations become an issue in Cached Exchange Mode.

Virtualized or Remote Desktop Services (Terminal Services) environments that run Outlook 2007 or Outlook 2003. Cached Exchange Mode is not supported when you run Outlook 2007 or Outlook 2003 on a computer running Remote Desktop Services (Terminal Services).

Outlook 2010 supports running in Cached Exchange Mode in a Remote Desktop Services (Terminal Services) environment that has multiple users. When you configure a computer running Remote Desktop Services (Terminal Services) to use Cached Exchange Mode, you must consider additional storage space that is required and disk I/O requirements of multiple client access.

By default, new Exchange accounts that are set up on a computer running Remote Desktop Services (Terminal Services) will use Online Mode. Upon setup, the user can decide to enable Cached Exchange Mode or this setting can be controlled by using the Use Cached Exchange Mode for new and existing Outlook profiles option in the Office Customization Tool or Group Policy.

In very limited bandwidth environments, Cached Exchange Mode can be configured to download only e-mail headers and a 256-character preview of the message body. For more information, see Configure Cached Exchange Mode in Outlook 2010.

Even when it is configured in Cached Exchange Mode, Outlook 2010 must contact the server directly to do certain operations. These operations will not function when Outlook is not connected and can take longer to complete on high-latency connections. These operations include the following:

By caching the user's mailbox and the OAB locally, Outlook no longer depends on continuous network connectivity for access to user information. While connected, Outlook continuously updates users’ mailboxes so that the mailboxes are kept up to date. If a user disconnects from the network — for example, by removing a portable computer, such as a laptop, from a docking station — the latest information is automatically available offline.

In addition to using local copies of mailboxes to improve the user experience, Cached Exchange Mode optimizes the type and amount of data sent over a connection with the server. For example, if the On slow connections, download only headers setting is configured in the Office Customization Tool, Outlook changes the type and amount of data sent over the connection.

Note

Outlook checks the network adapter speed on the user's computer to determine a user's connection speed, as supplied by the operating system. Reported network adapter speeds of 128 kilobytes (KB) or lower are defined as slow connections. Under some circumstances, the network adapter speed might not accurately reflect data throughput for users. For more information about adjusting the behavior of Outlook in these scenarios, see Managing Outlook behavior for perceived slow connections later in this article.

Outlook can adapt to changing connection environments by offering different levels of optimization, such as disconnecting from a corporate local area network (LAN), going offline, and then re-establishing a connection to the server over a slower, dial-up connection. As the Exchange Server connection type changes — for example, to LAN, wireless, cellular, or offline — transitions are seamless and do not require changing settings or restarting Outlook.

For example, a user might have a portable computer at work with a network cable connection to a corporate LAN. In this scenario, the user has access to headers and full items, including attachments. The user also has quick access and updates to the computer that runs Exchange Server. If a user disconnects the portable computers from the LAN, Outlook switches to Trying to connect mode. The user can continue to work uninterruptedly with the data in Outlook. If a user has wireless access, Outlook can re-establish a connection to the server and then switch back to Connected mode.

If the user later connects to the Exchange Server computer over a dial-up connection, Outlook recognizes that the connection is slow and automatically optimizes for that connection by downloading only headers and by not updating the OAB. In addition, Outlook 2010 and Office Outlook 2007 include optimizations to reduce the amount of data that is sent over the connection. The user does not need to change settings or restart Outlook in this scenario.

Outlook 2010 also includes the Need Password mode. A Need Password message is displayed when Outlook is in a disconnected state and requires user credentials to connect; for example, when a user clicks Cancel in a credentials authentication dialog box. When Outlook is disconnected but is not offline, a user-initiated action (such as clicking Send/Receive or the Type Password button on the ribbon) causes Outlook to prompt again for the password and to display a Trying to connect message until the user can successfully authenticate and connect.

Some Outlook features reduce the effectiveness of Cached Exchange Mode because they require network access or bypass Cached Exchange Mode functionality. The primary benefit of using Cached Exchange Mode is that the user is shielded from network and server connection issues. Features that rely on network access can cause delays in Outlook responsiveness that users would not otherwise experience when they use Cached Exchange Mode.

The following features might rely on network access and can cause delays in Outlook unless users have fast connections to Exchange Server data:

Delegate access, when folders are not cached locally (local cache is the default).

Opening another user's calendar or folder that is not cached locally (local cache is the default).

We recommend that you disable or do not implement the following features, or combination of features, if you deploy Cached Exchange Mode:

The toast alert feature with digital signatures on e-mail messages Outlook must check a server to verify a digital signature. By default, when new messages arrive in a user's Inbox, Outlook displays a toast message that contains a part of an e-mail message. If the user clicks the toast message to open a signed e-mail message, Outlook uses network access to check for a valid signature on the message.

Multiple Address Book containers The Address Book typically contains the global address list (GAL) and user Contacts folders. Some organizations configure subsets of the GAL, which display in the Address Book. These subset address books can also be included in the list that defines the search order for address books. If subset address books are included in the search order list, Outlook might need to access the network to check these address books every time that a name is resolved in an e-mail message that a user is composing.

Custom properties on the General tab in Properties dialog box for users The Properties dialog box appears when you double-click a user name (for example, on the To line of an e-mail message). This dialog box can be configured to include custom properties unique to an organization, such as a user's cost center. However, if you add properties to this dialog box, we recommend that you not add them to the General tab. Outlook must make a remote procedure call (RPC) to the server to retrieve custom properties. Because the General tab shows by default when the Properties dialog box is accessed, an RPC would be performed every time that the user accessed the Properties dialog box. As a result, a user who runs Outlook in Cached Exchange Mode might experience noticeable delays when he or she accesses this dialog box. To help avoid such delays, you create a new tab on the Properties dialog box for custom properties, or include custom properties on the Phone/Notes tab.

Certain Outlook add-ins can affect Cached Exchange Mode. Some add-ins can access Outlook data by using the object model to bypass the expected functionality of the Download only headers and On slow connections, download only headers settings in Cached Exchange Mode. For example, full Outlook items, not only headers, download if you use Microsoft ActiveSync technology to synchronize a hand-held computer, even over a slow connection. In addition, the update process is slower than if you download the items in Outlook, because one-time-only applications use a less-efficient kind of synchronization.

Cached Exchange Mode uses a local copy of the user’s Exchange mailbox, and in some cases, you can improve the performance of cached mode for your whole organization or for a group of users; for example, users who work remotely.

Cached Exchange Mode works independently of existing Outlook Send/Receive actions to synchronize users' .ost and OAB files with Exchange Server data. Send/Receive settings update users' Outlook data in the same way the settings did in earlier versions of Outlook.

Users who have Send/Receive-enabled Exchange accounts and who synchronize Outlook data by pressing F9 or by clicking Send/Receive might not realize that manual synchronization is no longer necessary. In fact, network traffic and server usage can be adversely affected if users repeatedly execute Send/Receive requests to Exchange Server. To minimize the effects, inform users that manual Send/Receive actions are unnecessary in Cached Exchange Mode. This might be especially helpful for remote users who typically used Outlook in offline mode with earlier Outlook versions and used Send/Receive to synchronize the data or just before they disconnected from the network. This kind of data synchronization now occurs automatically in Cached Exchange Mode.

Another way to manage the issue is to disable the Send/Receive option for users. However, we do not recommend this because it can create problems for some users; for example, when you upgrade current Outlook users with POP accounts and existing customized Send/Receive groups to Outlook 2010. In this situation, if you disable the Send/Receive option, users cannot download POP e-mail messages or HTTP e-mail messages by using the Outlook Connector.

Cached Exchange Mode enables Outlook to access the local Offline Address Book (OAB) for user information, instead of requesting the data from Exchange Server. Local access to user data greatly reduces the need for Outlook to make RPCs to the Exchange Server computer, and lessens much of the network access that is required for users in Exchange online mode or in previous versions of Outlook.

When users have a current OAB installed on their computers, only incremental updates to the OAB are needed to help prevent unnecessary server calls. Outlook in Cached Exchange Mode synchronizes the user's OAB with updates from the Exchange Server copy of the OAB every 24 hours. You can help control how often users download OAB updates by limiting how often you update the Exchange Server copy of the OAB. If there is no new data to synchronize when Outlook checks, the user's OAB is not updated.

Note

We recommend that users use the default Unicode OAB. The ANSI OAB files do not include some properties that are in the Unicode OAB files. Outlook must make server calls to retrieve required user properties that are not available in the local OAB, which can result in significant network access time when users do not have a Full Details OAB in Unicode format.

When you deploy Cached Exchange Mode for Outlook, be aware that users' local .ost files can increase 50 percent to 80 percent over the size of the mailbox reported in Exchange Server. The format Outlook uses to store data locally for Cached Exchange Mode is less space-efficient than the server data file format. This results in the use of more disk space when mailboxes are downloaded to provide a local copy for Cached Exchange Mode.

When Cached Exchange Mode first creates a local copy of a user's mailbox, the user's current .ost file, if one exists, is updated. If users currently have non-Unicode ANSI-formatted .ost files, we recommend that you upgrade their .ost files to Unicode. Non-Unicode (ANSI) Outlook files have a limit of 2 gigabytes (GB) of data storage. The maximum size for Unicode .ost files is configurable, with the default being 50 GB of data storage.

Also, make sure that users' .ost files are located in a folder that has sufficient disk space to accommodate users' mailboxes. For example, if users' hard drives are partitioned to use a smaller drive for system programs (the system drive is the default location for the folder that contains the .ost file), specify a folder on another drive that has more disk space as the location of users' .ost files.

By default, if a user’s Microsoft Outlook 2010 profile is configured in Cached Exchange mode, all items in the folders that are accessible by the Outlook profile in the shared mailbox are downloaded to the user’s local cache. This includes any other mailboxes or shared folders the user has added to the profile. This is a change from Microsoft Office Outlook 2007, in which only shared non-mail folder items are cached by default.

Cached Exchange Mode can be configured to download and synchronize the public folders included in users' Favorites folders for Outlook Public Folders. By default, Public Folder Favorites are not synchronized. However, you might want to enable this option if your organization uses public folders extensively. You can configure an option to download Public Folder Favorites in the .ost when you customize your Cached Exchange Mode deployment.

If users' Public Folders Favorites folders include large public folders, their .ost files can also become large. This can adversely affect Outlook performance in Cached Exchange Mode. Before you configure Cached Exchange Mode to enable this option, ensure that users are selective about the public folders that are included in their Public Folder Favorites. Also, ensure that users' .ost files are large enough, and are in folders that have sufficient disk space, to accommodate the additional storage requirements for the public folder downloads.

Outlook is configured to determine a user's connection speed by checking the network adapter speed on the user's computer, as supplied by the operating system. If the reported network adapter speed is 128 KB or lower, the connection is defined as a slow connection.

When a slow connection to an Exchange Server computer is detected, Outlook helps users have a better experience if they reduce the amount of less-critical information that is synchronized with the Exchange Server computer. Outlook makes the following changes to synchronization behavior for slow connections:

Switches to downloading only headers.

Does not download the Offline Address Book or OAB updates.

Downloads the body of an item and associated attachments only when it is requested by the user.

Outlook continues to synchronize the Outlook data with mobile devices, and some client-side rules might run.

Note

We recommend that you do not synchronize mobile devices with the Cached Exchange Download only headers setting enabled. When you synchronize a mobile device — for example, by using ActiveSync — full items are downloaded in Outlook, and the synchronization process is less efficient than with regular Outlook synchronization to users' computers.

The Download only headers setting for synchronization is designed for Outlook users who have dial-up connections or cellular wireless connections, to minimize network traffic when there is a slow or expensive connection.

Under some circumstances, the network adapter speed might not accurately reflect data throughput for users. For example, if a user's computer is connected to a local area network (LAN) for fast access to local file servers, the network adapter speed is reported as fast because the user is connected to a LAN. However, the user's access to other locations on an organization's network, including the Exchange Server computer, might use a slow link, such as an ISDN connection. For such a scenario, where users' actual data throughput is slow although their network adapters report a fast connection, you might want to configure an option to change or lock down the behavior of Outlook; for example, by disabling automatic switching to downloading only headers by using the Group Policy Object Editor option, Disallow On Slow Connections Only Download Headers. Similarly, there might be connections that Outlook has determined are slow but which provide high data throughput to users. In this case, you might also disable automatic switching to downloading only headers .

You can configure the On slow connections, download only headers option in the OCT, or lock down the option by using Group Policy Object Editor to set Disallow On Slow Connections Only Download Headers. For more information about how to customize this setting, see Configure Cached Exchange Mode in Outlook 2010.

Stage the rollout over time if you plan to upgrade a large group of users from a deployment of Outlook without Cached Exchange Mode to Outlook 2010 with Cached Exchange Mode enabled. Outlook without Cached Exchanged Mode is the case for Outlook 2002 or earlier, or Office Outlook 2003, or for Office Outlook 2007 without Cached Exchange Mode installed. A staged rollout over time helps your organization's Exchange Server computers manage the requirements of creating or updating users' .ost files.

Caution

If most user accounts are updated to use Cached Exchange Mode at the same time and then start Outlook at the same time (for example, on a Monday morning after a weekend upgrade), the Exchange Server computers have significant performance issues. These performance issues can sometimes be reduced; for example, if most of the users in your organization have current .ost files. But in general, we recommend staging deployment of Cached Exchange Mode over a period of time.

The following scenarios include examples of how you can deploy Cached Exchange Mode to avoid a large initial performance impact on the Exchange Server computers and, in some cases, minimize the time users spend waiting for the initial synchronization:

Retain Outlook .ost files when you deploy Cached Exchange Mode. Because existing .ost files are merely updated with the latest mailbox information when Outlook with Cached Exchange Mode starts for the first time, retaining these .ost files when you deploy Cached Exchange Mode can help reduce the load on your organization's Exchange Server computers. Users who already have .ost files will have less Outlook information to synchronize with the server. This scenario works best when most users already have .ost files that have been synchronized recently with Exchange Server. To retain .ost files while you deploy Outlook with Cached Exchange Mode, do not specify a new Exchange Server computer when you customize Outlook profile information in the OCT. Or, when you customize Outlook profiles in the OCT, clear the Overwrite existing Exchange settings if an Exchange connection exists (only applies when modifying the profile) check box. (If you specify an Exchange Server computer when you configure and deploy Outlook with this option enabled, Outlook replaces the Exchange service provider in the MAPI profile, which removes the profile's entry for existing .ost files.) If you are currently using non-Unicode (ANSI) .ost files, we recommend that you upgrade users’ .ost files to Unicode for improved performance and functionality. In this case, the old non-Unicode (ANSI) .ost files cannot be retained; they would be re-created in the Unicode format.

For information about how to force an upgrade of an existing non-Unicode (ANSI) formatted .ost file to Unicode format, see “Force upgrade of non-Unicode ANSI format .ost files to Unicode” in Configure Cached Exchange Mode in Outlook 2010.

Provide seed .ost files to remote users, and then deploy Cached Exchange Mode after users have installed the .ost files that you provide. If most users in your organization do not currently have .ost files or are not using Cached Exchange Mode, you can deploy Outlook 2010 with Cached Exchange Mode disabled. Then, before the date on which you plan to deploy Cached Exchange Mode, you provide initial, or seed, .ost files to each user with a snapshot of the user's mailbox; for example, by providing or mailing to the user a CD that contains the file together with installation instructions. You might also want to provide a recent version of your organization's Office Address Book (OAB) with Full Details. You configure and deploy Cached Exchange Mode when users confirm that they have installed the files.

When you update your Outlook deployment to use Cached Exchange Mode later, Exchange Server updates users' existing .ost files and there is much less data to synchronize than there would be if a new .ost file and OAB were created for each user. To create individual CDs for each user's .ost file can be time-consuming. Therefore, this seed-file deployment option might be most useful for select groups of remote users who would otherwise spend lots of time waiting for the initial mailbox and OAB synchronization, perhaps at a high cost, depending on their remote connection scenario.

Deploy Outlook with Cached Exchange Mode to groups of users over time. You can balance the workload on the Exchange Server computers and the local area network by upgrading groups of users to Cached Exchange Mode over time. You can reduce the network traffic and server-intensive work of populating .ost files with users' mailbox items and downloading the OAB by rolling out the new feature in stages. The way that you create and deploy Cached Exchange Mode to groups of users depends on your organization's usual deployment methods. For example, you might create groups of users in Microsoft Systems Management Server (SMS), to which you deploy a SMS package that updates Outlook to use Cached Exchange Mode. You deploy SMS to each group over a period of time. To balance the load as much as you can, choose groups of users whose accounts are spread across groups of Exchange Server computers.

The process of upgrading users to Outlook 2010 with Cached Exchange Mode already enabled in Office Outlook 2003 or Outlook 2007 is straightforward. If you do not change Cached Exchange Mode settings, the same settings are kept for Outlook 2010. There is no change to the .ost or OAB file format, and you do not have to re-create these files during an upgrade.

However, be aware that in Outlook 2010, by default, shared mail and non-mail folders that users access in other mailboxes are downloaded and cached in the user’s local .ost file when Cached Exchange Mode is enabled. This behavior differs from Outlook 2007 in which only shared non-mail folders are cached by default. Therefore, when a user's existing Office Outlook 2003 profile has Cached Exchange Mode enabled and that profile is upgraded to Outlook 2010, shared mail and non-mail folders are downloaded when the user accesses them. This can be problematic if your organization uses shared folders extensively and if you have .ost files that are close to the file limit (2 GB for ANSI .ost files and 20 GB by default for Unicode .ost files). When these factors are both present, you may experience performance issues and other problems if you download shared folders. For more information about file size limits, see The file size limits of .pst and .ost files are larger in Outlook 2010 (http://go.microsoft.com/fwlink/p/?LinkId=223378).

For new Outlook 2010 profiles or for upgrading existing Office Outlook 2003 or Outlook 2007 profiles, you can use the OCT or Group Policy to disable the download shared folder option and therefore help prevent problems with downloading shared folders. Be aware that the setting Download non-mail shared folders applies to both mail and non-mail folders in Outlook 2010.

In addition, be aware that caching for shared folders works differently from other caching for Cached Exchange Mode. For shared folders, replication to the local .ost file starts only when the user clicks the shared folder. After a user starts caching for the folder by clicking it, Outlook updates the folder in the same way as it does other Outlook folders that are synchronized in Cached Exchange Mode. However, if the user does not go to the folder at least one time every 45 days (the default value), the locally cached data for the folder is removed from the .ost file and will be not be downloaded until the user clicks on the folder again.

To configure the number of days when cached shared data is removed from the .ost file, you can configure the Synchronizing data in shared folders option in the OCT or Group Policy. For more information about how to configure Cached Exchange Mode, see Configure Cached Exchange Mode in Outlook 2010.

Some Outlook users who connect to Exchange Server in online mode might have .ost files. If these users have a non-Unicode (ANSI) formatted .ost file and large Exchange mailboxes, they might experience errors when Outlook attempts to synchronize their mailboxes to their .ost files. We recommend that you upgrade users’ .ost files to the Unicode format as Outlook Unicode files do not have the 2-GB size limit that Outlook ANSI files do. Unicode is the default file format for Outlook 2010. For information about how to force an upgrade of an existing non-Unicode (ANSI) formatted .ost file to Unicode format, see To force upgrade of non-Unicode ANSI format .ost files to Unicode in Configure Cached Exchange Mode in Outlook 2010.

You can lock down the settings to customize Cached Exchange Mode by using the Outlook Group Policy Administrative template (Outlk14.adm). Or, you can configure default settings by using the Office Customization Tool (OCT), in which case users can change the settings.

By using Group Policy, you can help prevent users from enabling Cached Exchange Mode in Outlook 2010, and you can enforce download options for Cached Exchange Mode or configure other Cached Exchange Mode options. For example, you can specify the default times between Exchange Server synchronizations when data changes on an Exchange Server computer or on the client computer.

The following table shows some of the settings that you can configure for Cached Exchange Mode. In Group Policy, the settings are found under User Configuration\Administrative Templates\Microsoft Outlook 2010\Account Settings\Exchange\Cached Exchange Mode. The OCT settings are in corresponding locations on the Modify user settings page of the OCT.

Option

Description

Disallow Download Full Items

Enable to turn off the Download Full Items option in Outlook. To find this option, click the Send/Receive tab, and then click Download Preferences.

Disallow Download Headers

Enable to turn off the Download Headers option in Outlook. To find this option, click the Send/Receive tab.

Disallow Download Headers then Full Items

Enable to turn off the Download Headers then Full Items option in Outlook. To find this option, click the Send/Receive tab, and then click Download Preferences.

Disallow On Slow Connections Only Download Headers

Enable to turn off the On Slow Connections Download Only Headers option in Outlook. To find this option, click the Send/Receive tab, and then click Download Preferences.

Download Public Folder Favorites

Enable to synchronize Public Folder Favorites in Cached Exchange Mode.

Enable to configure new and existing Outlook profiles to use Cached Exchange Mode. Disable to configure new and existing Outlook profiles to use Online Mode.

The following table shows some additional settings that you can configure for Exchange connectivity. In Group Policy, the settings are found under User Configuration\Administrative Templates\Microsoft Outlook 2010\Account Settings\Exchange. The OCT settings are in corresponding locations on the Modify user settings page of the OCT.

For more information about how to plan a Cached Exchange Mode deployment, see the following resources.

When you use Office Outlook 2003, Office Outlook 2007, or Outlook 2010 with Exchange Server-based systems, you can use Cached Exchange Mode and other features to enhance the user experience regarding issues such as high latency, loss of network connectivity, and limited network bandwidth. To learn about these improvements, see Client Network Traffic with Exchange 2003 white paper (http://go.microsoft.com/fwlink/p/?LinkId=79063).